Activated sludge systems for treatment of municipal and industrial wastewaters are well known in the art. In the older and more conventional of these systems the wastewater, usually after an initial primary sedimentation for removal of contained solids, is subjected to oxidative treatment with atmospheric air or a gas stream of higher oxygen concentration in one or more treating stages, in the presence of biomass obtained by secondary clarification or sedimentation of the oxidized liquid product to separate out a dense solids-containing fraction, constituting the activated sludge recycled to the oxidative treatment for admixture with the wastewater influent.
In addition to methods employing chemical precipitation of phosphates for their removal from treated wastewaters, attempts were made to effect removal of phosphates by modification of operating conditions in conventional aerobic activated sludge wastewater treating systems with or without inclusion of a chemical precipitation step. Thus, in Levin, U.S. Pat. No. 3,236,766, the suggested process principally involves mixing the wastewater influent with part of the recycled sludge and subjecting the obtained mixed liquor to oxygenation and aeration. Following froth floatation of the aerated mixed liquor, a separated sludge fraction is sent to a "phosphate stripper and sludge thickener" where it is adjusted to acidic conditions and is maintained "in a non-aerated or anaerobic condition for about 10 to 20 minutes". As a result of these conditions, patentee indicates, considerable quantities of intracellular phosphate are induced to leak out of the sludge into the liquid phase which is withdrawn and subjected to chemical precipitation of phosphates therein. The phosphate-depleted sludge produced in the stripper-thickener is preferably re-aerated, the pH adjusted to the non-acid condition and introduced to the flash mixer for admixture with the wastewater influent. Among other alternative modifications suggested in the patent, the aerated mixed liquor may be sent to a settling basin and a portion of the settled sludge therefrom sent to the phosphate stripper-sludge thickener.
In U.S. Patents to G. V. Levin et al., U.S. Pat. Nos. 3,730,882 and 3,756,946, the aerated mixed liquor is passed to a settling zone containing an anaerobic layer of sludge into which layer the phosphate, which was taken up by the cells of the biomass during the aeration stage, is settled and a substantially phosphate-free effluent is removed from above the sludge layer. By maintaining the sludge layer under anaerobic conditions for sufficient time, according to the U.S. Pat. No. 3,730,882, the organisms therein are caused to release phosphate to the liquid phase. A phosphate precipitant is added to precipitate the soluble phosphate and phosphate-depleted sludge containing precipitated phosphate particles is recycled for admixture with the influent sewage entering the aeration zone. According to the U.S. Pat. No. 3,756,946, sludge containing soluble phosphate in the liquid phase is removed from the settling zone and then treated to separate out a phosphate-enriched aqueous phase and to provide a sludge having a higher concentration of solids with a lower phosphate concentration, which sludge is recycled for admixture with the sewage influent going to the aeration zone.
In U.S. Pat. No. 4,183,808 to Drnevich, a variation of the scheme of Levin is described in which sludge removed from the settling zone is treated to the extent necessary to release into the liquid phase in soluble form phosphorus contained in the biomass. The solubilized phosphate containing sludge is then recycled for admixture with sewage influent going to the aeration zone.
According to Casey et al. U.S. Pat. No. 3,994,802, nitrogenous pollutants in addition to carbonaceous BOD are removed from wastewater in a multistage activated sludge treating process, wherein the recycled sludge is admixed with the influent wastewater in the presence of sufficient oxygen to maintain oxic conditions and the treated mixed liquor from the oxic stage is passed to an anoxic stage wherein the NO.sub.x.sup.- (nitrates and nitrites), formed by the oxidation of ammonium compounds, is reduced to nitrogen gas, which is discharged. After one or more of such successive oxic and anoxic treating stages, the mixed liquor is subjected to solids separation, and the separated solids fraction, constituting the activated sludge, is recycled to at least the initial mixing stage. A short residence time, resulting in a high food to biomass ratio, is maintained in the initial oxic stage or the first subsection thereof, to avoid sludge bulking and to promote production of an active dense biomass. The patent cites earlier nitrification-denitrification techniques described in the literature.
As employed in said U.S. Pat. No. 3,994,802, the term "anoxic" refers to a condition of low oxygen tension, under which condition nitrates and nitrites are reduced to nitrogen gas. The term "oxic" refers to conditions of sufficiently high oxygen tension whereby nitrifying bacteria present in activated sludge will convert nitrogenous compounds into nitrates and/or nitrites.
The effective removal of contained phosphates from wastewater in a modified activated sludge process is disclosed in Spector U.S. Pat. No. 4,056,465. According to this patent the BOD-containing wastewater and recycled sludge are initially admixed under anaerobic conditions in the substantial absence of oxygen or oxidizing agents, followed by aeration, and subsequent clarification to separate out the activated sludge for recycle. The patented operation is designed to promote selective production of nonbulking biomass while obtaining desired high removal of phosphates. By an alternative modification described in said patent, nitrates and nitrites are also removed from the wastewater by interposing an anoxic treating zone between the anaerobic zone and the aerating zone. The pertinent disclosure of U.S. Pat. No. 4,056,465 is incorporated herein by reference thereto.
Process of the type described in said U.S. Pat. No. 4,056,465, employing in an activated sludge wastewater treating process an anaerobic mixing and treating zone followed by an oxic aerating zone, have become known as A/O systems, whereas those systems having an intermediate anoxic treating zone between the anaerobic and oxic zones are referred to as "A/A/O" or "A.sup.2 /O" systems.
Further improvements in systems of the A/O and A.sup.2 /O type are disclosed in U.S. Pat. No. 4,271,026. By maintaining both the F/M ratio and the BOD/P ratio within particular ranges therein set out and preferably relative to one another, enhanced phosphorus removal is assured while operating at an adequately high throughput rate. The initial admixing of the wastewater influent and recycled sludge is carried out under strictly anaerobic conditions, in the substantial absence of oxygen and oxidizing agents, such that the mixed liquor has a D.O. content of less than about 0.7 ppm. In the subsequent oxygenation treatment a dissolved oxygen content of at least 1 ppm is maintained. Within the entire process system encompassing the initial anaerobic treatment and extending through the oxygenation, including any intervening anoxic treating step if used, the BOD/P ratio is maintained from about 5:1 up to about 50:1 and the food to biomass (F/M) ratio from about 0.09 up to an upper limit of about 1.4.
In an article by Davelaar et al., titled "The Significance of an Anaerobic Zone for the Biological Removal of Phosphate from Wastewaters" (Water, S. A. Vol. 4, No. 2, April 1978, pages 54 to 60), the authors describe certain of the theories advanced with respect to the function of the anaerobic zone, particularly with respect to phosphate removal and the adverse effect on phosphate removal by the presence of nitrate in the inflow to the anaerobic zone. The article compares experimental results obtained in the laboratory scale operation of two activated sludge units designated A and B. In the B unit the sewage influent and recycled sludge were admixed in an anoxic zone and flowed therefrom to an aerobic zone, followed by solids separation to recycle the settled activated sludge fraction to the anoxic zone. In the A unit an anaerobic zone was interposed between the anoxic and aerobic zones. Unit A, with the interposed anaerobic stage, was found to have superior phosphate removal ability.
In previous laboratory and pilot plant studies leading to the development of the present invention, it was observed that the introduction of nitrates and/or nitrites (NO.sub.x.sup.-) in the anaerobic zone of an A/O system interfered with the phosphorus-removing organisms, as a result of which there is an overall reduced level of P removal. Unless resort is had to the use of specific nitrification inhibitors, the A/O system would have to be operated at a high F/M in order to preclude growth of nitrifying bacteria as is known to the art, or operation with a sludge retention time in the clarifier sufficient to assure nitrification prior to admission of recycle sludge to the initial anaerobic zone. In this latter case there is the possibility of producing sufficient bubbles to elemental nitrogen by denitrification of NOX such that settled sludge may be carried to the surface of the clarifier by rising bubbles of nitrogen gas. This phenomenon produces unsatisfactory total suspended solids level.
As herein employed the terms "anaerobic", "anoxic" and "oxic" have the following definitions:
"Anaerobic" refers to a state existing within a wastewater treating zone which is substantially free of NO.sub.x.sup.- and the dissolved oxygen level (DO) is less than 0.7 ppm.
"Anoxic" refers to the state maintained in a wastewater treating zone wherein nitrates and/or nitrites are utilized to metabolize BOD, while dissolved oxygen levels are maintained at below 0.7 ppm. The nitrates and/or nitrites are thereby reduced to elemental N.sub.2 gas.
"Oxic" refers to that state in a wastewater treating zone wherein oxygen-containing gas is utilized for oxygenation therein and the dissolved oxygen level is maintained at above about 1 ppm.